1. Field of the Invention
The present invention relates generally to motor control systems. In particular, the invention is an improvement to a closed loop control circuit for a brushless motor.
2. Description of the Prior Art
Various circuits exist in the prior art for the commutation of currents in the motor windings of brushless DC motors. In a typical drive circuit, the windings in the brushless motor are divided into three or more segments. The windings are each driven by semiconductor controls or inverter circuits and the inverter stages are, in turn, driven by a commutation circuit which receives inputs from a commutation encoder connected to the shaft of the motor.
Because the voltage drive applied to the windings of the brushless motor is only applied during a fraction of a motor rotation as shown in FIG. 1, the resultant drive current on any winding of the motor is quite discontinuous and includes numerous components having higher harmonic frequencies than the frequency corresponding to the period of the motor. Some prior art circuits have modified the voltage drive command signal to approach a drive current. Typically, these arrangements have superimposed a high frequency oscillation on the alternating drive command voltage signal pulse width modulated signal which is, in turn, used to drive the output stage of the inverter to form a sinusoidal drive current having a generally triangular wave higher frequency signal superimposed upon it. That technique, however, does not provide results which are satisfactory at all operating velocities of the motor. Because of the substantial motor inductance, the phase angle between the drive voltage and motor current begins to shift or lag at higher velocity and the generated high speed torques is reduced.
In the prior art system as described in an article by James R. Woodbury entitled "The Design of Brushless DC Motor Systems," published in May 1974 in IEEE Transactions on Industrial Electronics and Control Instrumentation, and as discussed in U.S. Pat. No. 4,447,771 to Whited, the phase of a sinusoidal command voltage is shifted linearly in accordance with the motor velocity to adjust the motor torque angle. Even with such torque angle control, however, the system does not provide uniform results and maximum torques at all speeds because the drive current and hence the motor torque is out of phase. It also provides no compensation for motors having a trapezoidal field distribution.
Because prior conventional compensation circuitry fails to provide adequate compensation of the command signal shape and phase to provide a relatively sinusoidal drive current and a sinusoidal torque distribution at all operating frequencies and torque loads, prior art techniques have not provided optimum current vector control. Further compensation of the drive signal phase and shape is necessary to provide optimal performance of a brushless motor servo system.